Data which is recorded on an optical disk is reproduced by irradiating the rotating optical disk with a relatively weak light beam of a constant light amount, and detecting reflected light that has been modulated by the optical disk.
On a read-only optical disk, pits of information are pre-recorded in a spiral manner during the production of the optical disk. On the other hand, in the case of a rewritable optical disk, a film of recording material which permits optical data recording/reproduction is deposited on the surface of a base material in which spiral tracks having lands or grooves are formed, by using a method such as vapor deposition. In the case where data is to be recorded on a rewritable optical disk, the optical disk is irradiated with a light beam whose light amount is modulated in accordance with the data to be recorded, thus causing local changes in the characteristics of the recording material film and effecting a data write.
Note that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are smaller than the thickness of the optical disk base material. Therefore, a portion of an optical disk where data is recorded constitutes a two-dimensional surface, which may sometimes be referred to as an “information recording plane”. By taking note of the fact that such an information recording plane has a physical dimension also along the depth direction, the present specification employs the term “information recording layer”, instead of “information recording plane”. An optical disk includes at least one such information recording layer. Note that, in reality, a single information recording layer may include a plurality of layers, such a layer of phase-changeable material, a reflective layer, etc.
When recording data on a recordable optical disk, or reproducing data which is recorded on such an optical disk, it is necessary that the light beam always has a predetermined convergence state on a target track of the information recording layer. This requires “focus control” and “tracking control”. “Focus control” refers to controlling the position of an objective lens along a normal direction of the information recording layer (hereinafter referred to as the “depth direction of the substrate”) so that the focal point of the light beam will always be positioned on the information recording layer. On the other hand, tracking control refers to controlling the position of an objective lens along a radial direction of the optical disk (hereinafter referred to as the “disk radial direction”) so that the light beam spot will always be positioned on a predetermined track.
As conventional high-density/large-capacity optical disks, optical disks such as DVD (Digital Versatile Disc)-ROMs, DVD-RAMs, DVD-RWs, DVD-Rs, DVD+RWs, and DVD+Rs have been put to practical use. In addition, CDs (Compact Discs) are still in use. Currently, next-generation optical disks which have a higher density and a larger capacity than those of the above optical disks are being developed and put to practical applications, e.g., Blu-ray Discs (BDs). Moreover, in order to increase the capacity of data which can be recorded on a single optical disk, optical disks having a plurality of stacked information recording layers are also being developed.
Among the aforementioned optical disks, some are used in a bare state, rather than being accommodated within a cartridge. Flaws are likely to be formed and dust particles or fingerprints are likely to adhere on the surface of such an optical disk. The scratches, dust particles, and fingerprints on the surface of an optical disk behave as optical obstacles against a light beam with which the optical disk is irradiated. As a result, servo for focus control or tracking control may fail, or the amplitude of the readout signal (RF signal) may become too small, so that data recording/reproduction can no longer be stably performed.
Patent Document 1 discloses a technique of detecting scratches which are formed on an optical disk, and reducing the gains of focus servo and tracking servo while the light beam reflection is under the influence of scratches. By reducing the servo gains, the influence of scratches can be compensated for.
The technique disclosed in Patent Document 1 adopts a three beam method, where side lobes which occur due to a diffraction phenomenon caused by super resolution artifacts of a main beam are utilized. In this technique, a side lobe will move over an optical disk before a main beam does. When this side lobe passes over a scratch on the optical disk, the intensity of the reflected light increases due to the scratch. By detecting such a change in intensity, it becomes possible to reduce the gain immediately before the main beam, which follows the side lobe, passes over the scratch.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 8-235586 (paragraphs 2 to 18, FIG. 1, FIG. 4)